Preheat oven to 325 degrees F.
In a medium saucepan over low heat, melt the butter and unsweetened chocolate, stirring often.
Remove from the heat and stir in the sugar, then the eggs and the vanilla.
Mix in the flour.
Scrap the batter into a prepared 8-inch pan (sprayed with baking spray); smooth the top, sprinkle chocolate chips and walnuts on top. Bake for 25 minutes.
Right when brownies come out press the marshmallows and allow them to melt into the brownies. Put as many or as little as you like. Let cool on a wire rack.
These brownies are more like flourless cake and are nice and dense.